I am developing an altcoin using a new PoW algorithm which is written in C to test the algorithm. I get compilation errors related to compiling the C code when trying to compile. How can I properly add the C code to the Makefile?

My fork is based of Bitcoin Core 0.12.1 I am compiling on Ubuntu 16.04

Here is the compilation error:

  CXXLD    bitcoind
  /usr/bin/ld: crypto/libbitcoin_crypto.a(crypto_libbitcoin_crypto_a- 
  rainforest.o): relocation R_X86_64_32S against `rf_crc32_table' can not be 
  used when making a shared object; recompile with -fPIC
  crypto/libbitcoin_crypto.a: error adding symbols: Bad value

Here is how I added the files to src/Makefile.am:

# crypto primitives library
crypto_libbitcoin_crypto_a_CPPFLAGS = $(AM_CPPFLAGS) 
$(BITCOIN_CONFIG_INCLUDES)
crypto_libbitcoin_crypto_a_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
crypto_libbitcoin_crypto_a_SOURCES = \
   crypto/common.h \
   crypto/hmac_sha256.cpp \
   crypto/hmac_sha256.h \
   crypto/hmac_sha512.cpp \
   crypto/hmac_sha512.h \
   crypto/ripemd160.cpp \
   crypto/ripemd160.h \
   crypto/sha1.cpp \
   crypto/sha1.h \
   crypto/sha256.cpp \
   crypto/sha256.h \
   crypto/sha512.cpp \
   crypto/sha512.h \
   crypto/rainforest/rainforest.c \
   crypto/rainforest/rainforest.h

The files are rainforest.c and rainforest.h

Sorry for this question, I am a newbie in C and C++ developing, I come from Java and Python

EDIT: I didn't mention before that I linked rainforest.h by doing this:

extern "C"{ 
#include "crypto/rainforest/rainforest.h"
}